SONET and SDH are equivalent standards (respectively American and European) for transporting data on optical networks. According to these standards, data corresponding to multiple communication channels are conveyed in hierarchical frames.
FIG. 1 shows an exemplary hierarchy according to the SONET standard. This hierarchy corresponds to the Optical Carrier level 3 (OC3) signal which has a 155.52 Mbits/s bit rate. The top-level frame STS3 (Synchronous Transport Signal level 3) is transmitted in 125 μs. It contains three STS1 frames. Each STS1 frame contains seven groups of Virtual Tributaries (VT). A group may include four VT1.5 containers (size 1.5 VTs). Each VT1.5 container includes 24 bytes corresponding to concurrent DS0 signals. A DS0 (Digital Signal level 0) has a data transmission rate of 64 kb/s.
The OC3 signal thus conveys 2016 concurrent DS0 signals and has an effective bit rate of 129.024 Mbits/s. The remainder of the 155.52 Mbit/s capacity is used for transporting necessary overhead.
Other hierarchies exist. In particular, the VT groups of an STS1 frame may contain, each, three VT2 frames, two VT3 frames or one VT6 frame.
FIG. 2 shows the SDH equivalent of the hierarchy of FIG. 1. The top level frame is STM1 (Synchronous Transport Module level 1). It contains three VC3 (Virtual Container level 3) frames. Each VC3 frame contains seven groups of three TU12 (Tributary Unit level 12) frames. Finally each TU12 frame contains 32 DS0 bytes. This also amounts to 2016 DS0 signals.
For conciseness, the following description will only refer to the SONET standard as an example, but the teachings provided herein readily apply to the SDH standard.
FIG. 3 shows the structure of an STS3 frame. It is arranged in a byte matrix of 270 columns and 9 rows. The bytes of the matrix are transmitted from left to right and from top to bottom. Each of the three conveyed STS1 frames has three columns of transport overhead. These overhead columns are interlaced in the nine first columns of the matrix. As an example, the first three transport overhead bytes, designated by A1, A2 and J0, are marked for the first STS1 frame (i.e. the bytes in the first row of the 1st, 4th and 7th columns).
The overhead bytes are fully documented in the standard. Only some of them will be mentioned herein for the comprehension of the invention.
The three STS1 frames, designated STS1-1, STS1-2 and STS1-3, are contained in respective remaining columns of the matrix, in interlaced manner, i.e., from the 10th to the 270th column, three consecutive columns belong respectively to the three STS1 frames.
The STS1 frames are conveyed in a floating manner: they start at arbitrary positions in the STS3 frame. Two bytes, designated by H1 and H2, of the transport overhead of each STS1 frame form a pointer indicating the position of the first byte of the STS1 frame. The first byte of an STS1 frame is a “path overhead” byte designated by J1.
One purpose of this floating frame and pointer technique is to allow STS1 frames to be inserted at any point in time into a current transmission. Another purpose is to compensate transmission speed drifts, especially drifts in clock rates between the transmitter and receiver ends. If the transmitter clock is slower than the receiver clock, a meaningless byte is inserted every now and then between consecutive STS1 frames, whereby the pointer to the second frame is incremented by 1. This is designated as positive stuffing in the standard.
If the transmitter clock is faster than the receiver clock, an STS1 frame is shifted back by one byte every now and then, whereby the pointer to the frame is decreased by 1. In fact, the STS1 frame will not overlap the previous frame, because a specific transport overhead byte (designated by H3) is provided to receive the first byte of the shifted back frame. This is designated as “negative stuffing” in the standard.
FIG. 4 shows the structure of an STS1 frame. It is arranged according to a byte matrix of 87 columns and 9 rows. The first column contains the “path overhead”, the first byte of which is designated by J1. Columns 30 and 59 contain fixed stuffing bytes. The remaining columns contain the 7 groups of 4 VT1.5 containers, interleaved by groups. These columns are designated in FIG. 4 by references of the type i-j, where i is the group number and j the frame number in the group.
The first row, between positions 2 and 29, contains overhead bytes, designated by V1, V2, V3 and V4, which are intended to identify the starting points of the VT1.5 containers, since such frames are also conveyed in a floating manner.
FIG. 5 illustrates how consecutive VT1.5 containers are transmitted and identified. In fact, four consecutive VT1.5 containers are conveyed in a super-frame comprised of four consecutive STS1 frames. Bytes V1 to V4 are transmitted respectively in the 1st to 4th STS1 frames of the super-frame. Bytes V1 and V2 form a “VT payload pointer” and identify the position of the first byte of the first of the four respective VT1.5 containers. This first byte is designated by V5. The first bytes of the three other VT1.5 containers are respectively designated by J2, Z6 and Z7. Bytes V5, J2, Z6 and Z7 constitute a “VT path overhead”.
Such a hierarchy is for example used for handling telephone links corresponding to DS0 signals in a telephone exchange. For this purpose, one uses add/drop multiplexers. Such a device extracts (drops) a specific DS0 data flow from an OC3 link and/or inserts (adds) a DS0 data flow into the OC3 link. The remaining traffic passes straight through the multiplexer without additional preprocessing.
FIG. 6 schematically shows a conventional add/drop multiplexer (ADM) 10. The ADM may be built around two integrated circuits made by PMC-Sierra, i.e. the PM5342 path/section transceiver, aka SPECTRA, and the PM5362 tributary unit payload processor (TUPP). The association of these circuits identifies the positions of the payloads of the VT1.5 containers in an OC3 link and thus allows these containers or their DS0 signals to be extracted individually in a simple manner. The insertion of VT1.5 containers in the OC3 link is straightforward, since the necessary position information has already been calculated for the extraction.
The individual DS0 signals (extracted or to insert) are exchanged over a standard telecommunications bus, such as SC, MUIP or H.100, to carry out any necessary operations, such as switching or routing. The DS0 signals of each VT1.5 container are exchanged with the telecommunications bus over an individual T1 link and bus interface 12.
Such add/drop multiplexers are usually limited to the processing of a few VT1.5 containers. Indeed, a single VT1.5 container conveys 24 DS0 signals which are statistically sufficient for satisfying 360 telephone subscribers. Therefore, just a few VT1.5 containers (up to eight) are necessary in most telephone exchanges. The processing of all the VT1.5 containers would require 84 bus interfaces 12, usually in the form of individual add-on cards, which would considerably increase the complexity and bulk of the device.
However, there is a growing need for the handling of large numbers of telephone communications from centralized locations. Such a need is found especially in large scale voice-mail systems.